Fastener Thread Machining Precision: 6g, 6h Tolerance Zones and Assembly Relations
Executive Summary
Thread tolerance zones 6g and 6h look like mere codes, but they actually determine whether a bolt threads in smoothly and whether the fit is tight or loose. This article explains the meaning of the three characters in a tolerance zone, the fit logic of fundamental deviations, the real difference between 6g and 6h, the effect of four form errors on assembly, and tolerance-based cause attribution for common assembly problems.
Table of Contents
- It Won't Thread In—Whose Problem Is It?
- How to Read a Tolerance-Zone Code
- The Real Difference Between 6g and 6h
- Fit Selection: Tighter Is Not Always Better
- Four Form Errors That Affect Assembly Precision
- Inspection Methods and Judgment of Common Problems
- Conclusion: The Tolerance Zone Is Part of the Design Language
It Won't Thread In—Whose Problem Is It?
One of the most common disputes on the assembly floor: "the bolt won't screw in." Purchasing blames the bolt supplier, assembly thinks the nut is at fault, each side produces its own inspection report, the GO gauges both pass, and the dispute goes unresolved.
The root cause of such problems is often not "whether it is qualified," but the fit selection of the tolerance zones. A thread is not simply "fits if dimensions are on spec"; it has a precise fit logic: external threads have their own tolerance zone, internal threads have theirs, and the limit dimensions of the two determine whether assembly goes smoothly, is tight, or simply cannot go together.
How to Read a Tolerance-Zone Code
A thread tolerance zone consists of three parts: tolerance grade + fundamental deviation + tolerance position. Taking 6g and 6h as examples:
- The number 6: tolerance grade (accuracy class); the smaller the number, the tighter the tolerance. External threads commonly use grades 4, 6, 8; internal threads commonly use grades 4, 5, 6, 7.
- The letter g: fundamental deviation, which determines the tolerance zone's position relative to the basic size. Lowercase letters are for external threads, uppercase letters for internal threads.
- Fit length: S (short), N (normal, usually omitted), L (long).
Meaning of Fundamental Deviations
| Code | Applies To | Tolerance Zone Position | Fit Nature |
|---|---|---|---|
| e | External thread | Away from the zero line, size on the small side | Loose fit, large clearance |
| f | External thread | Rather away from the zero line | Relatively loose fit |
| g | External thread | Slightly away from the zero line | Most common, with clearance, easy to assemble |
The Real Difference Between 6g and 6h
The two have the same tolerance grade (both grade 6); the difference lies only in the fundamental deviation:
| Item | 6g (External Thread) | 6h (External Thread) |
|---|---|---|
| Fundamental deviation | Negative (tolerance zone shifted down) | Zero (upper limit equals basic size) |
| Major diameter upper limit | Smaller than basic size | Equal to basic size |
| Fitting with 6H internal thread | With guaranteed clearance, assembles smoothly | Minimum clearance zero, assembly tighter |
| Plating adaptability | Good (clearance reserved for coating) | Poor (coating easily causes out-of-tolerance) |
Fit Selection: Tighter Is Not Always Better
The combination of external and internal thread tolerance zones determines the fit nature:
| Fit Combination | Fit Nature | Assembly Difficulty | Typical Application |
|---|---|---|---|
| 6H / 6g | Clearance fit (standard) | Easy | General fasteners, the most common combination |
| 6H / 6e, 6H / 6f | Larger clearance fit | Very easy | Thick coatings, frequent disassembly |
| 6H / 6h | Minimum clearance zero | Rather difficult | Precision machinery, high-strength fits |
Four Form Errors That Affect Assembly Precision
Even if dimensions are within tolerance, form errors can still cause assembly problems:
| Error Item | Definition | Typical Allowable Value | Effect on Assembly |
|---|---|---|---|
| Cumulative pitch error | Accumulation of pitch deviation over several threads | By tolerance grade | Uneven flank contact, higher assembly torque |
| Half-angle error | Flank angle deviating from the theoretical value | Per standard | Local interference, easy galling |
| Pitch diameter taper | Pitch diameter variation along the axis | Per standard | Thread gets tighter as it is turned in |
Of these, perpendicularity has the greatest effect on actual fastener use. If the head bearing face is not perpendicular to the thread axis, it causes: forced bolt bending, uneven circumferential distribution of preload, and additional bending stress on the bolt. This is also why GB/T 2-2016 makes explicit provisions for thread ends (chamfer, runout)—end shape directly affects the thread's initial lead-in.
Inspection Methods and Judgment of Common Problems
Common Gauges and Tools
| Inspection Item | Common Gauge / Method | Notes |
|---|---|---|
| Virtual (effective) pitch diameter | Thread GO gauge | Must freely thread in along the full length |
| Single-pitch diameter lower limit | Thread NO-GO gauge | Must not thread in more than 2 turns |
| Major diameter | Micrometer, optical projector | External thread major diameter |
| Pitch diameter (precise) | Three-wire method, thread micrometer, vision machine | For arbitration |
Common Assembly Problems and Tolerance Causes
| Symptom | Possible Cause | Check Direction |
|---|---|---|
| GO gauge won't enter | Pitch diameter too large, profile error, burrs | Measure pitch diameter, check profile, deburr |
| NO-GO gauge threads in too far | Pitch diameter too small, profile wear | Die plate wear, blank diameter |
| Difficult assembly threading | External/internal thread fit too tight | Verify fit combination, switch to 6g |
| Abnormally high torque after assembly | Rough thread surface, dimensional interference | Roughness, coating thickness |
A typical engineering problem: the GO gauge won't enter after electro-galvanizing. The cause is that the coating occupies the clearance in the thread tolerance zone. The solution is not "plate thinner," but changing the external thread tolerance zone from 6h to 6g or 6e on the drawing to reserve room for the coating. This is the most practical piece of experience in tolerance-zone selection.
Conclusion: The Tolerance Zone Is Part of the Design Language
The three characters of a thread tolerance zone convey a complete fit intent: accuracy grade, deviation direction, and fit length. It is not a labeling habit; it is design language.
For fasteners, the three most practical principles are: standard fasteners use 6g external thread with 6H internal thread; when coated, enlarge the clearance to reserve room for the coating; when strength is needed, rely on engagement length and preload rather than tightening the tolerance. Write these three into the drawings, and many disputes on the assembly floor disappear naturally.
